When someone speaks, acts or performs around others in a way that assumes the other people already know the basics. This can be fine for all parties involved, but it can also be considered rude, depending on what the subject matter is and who's receiving it.
A friend invites you over for dinner with his family. During dinner, the relatives start talking endlessly about Mortimer's problems. You keep thinking: "Who the fuck is Mortimer?" and no one at the table has the courtesy to explain to you who Mortimer is. The family was speaking "matter of factly" about some loser named Mortimer.
Snottrah: Do you want to come back over for dinner?
Perphenalia: Nah. Last time I was there everyone started talking matter-of-factly about some prick named Mortimer and I sat there like an idiot.
Snottrah: Oh, sorry. Mortimer's my meth addict uncle. We disowned him a couple of years back for holding himself at gunpoint and carjacking his own car.
Why is the professor talking so matter-of-factly about That's because you missed the first two classes, genius.
Phrase used by morons to transition from one confusing sentence clause to another. Often used when said moron has absolutely no clue what he or she is speaking about and is engaged in an argumentative conversion. When properly used, the moron--through his own stupidity--renders his conversing opponent speechless and effectively ends the conversation. Consequentially, the moron now believes he is smart and begins to use the phrase more often; making everyone who knows the moron hate him and dread talking to him.
reasonably intelligent person: "The consensus among dietitians is that you should eat solid food and that a diet consisting of mere protein shakes is a crash diet that cannot be sustained for long-term, healthy weight loss."
moron: "Well, I am no scientist and I sure as hell won't be called Tom Cruise, but the fact of the matter is I've lost ten pounds in two weeks on my shake diet and its totally kick ass. Do you even realize that I put ice in the shake and its a solid?"